The present invention relates to the field of the separation of gaseous mixtures by adsorption on a carbonated adsorbent, such as active carbon, with improved properties, in particular a PSA process to produce hydrogen.
A PSA unit for the purification of hydrogen contains an adsorbent or a combination of adsorbents which must be capable of selectively retaining the impurities contained in the gas to be treated.
The most common impurities of the gases containing hydrogen are: N2, CO2, CH4, CO, C2H4, C2H6, C3H8, C4+, BTX (benzene-toluene-xylene) compounds, water vapor, mercaptans, H2S, SO2.
These compounds are eliminated from the hydrogen flow by an assembly of adsorbents disposed in series.
Conventionally, alumina or silica gel essentially retain the water vapor; activated carbon is used to retain the heavy hydrocarbons, the CO2 and the water vapor; and zeolite is used to eliminate particularly the impurities N2, CO and CH4.
The proportion of the different adsorbents depends on the composition of the gas to be treated under pressure.
Thus, there are a great number of possible combinations of adsorbents, taking account of the exact nature of the adsorbents and their relative proportions.
The production of high purity hydrogen is of great interest in the industrial field, it being widely used in a number of synthesis processes such as hydrocracking, the production of methanol, the production of oxoalcohols and isomerization processes.
In the prior art, the PSA processes have been very effective for the separation of gaseous mixtures and particularly for the production of pure hydrogen or oxygen from gaseous mixtures contaminated by various impurities.
The PSA processes preferably use the selective adsorption of a given adsorbent for one or several of the contaminating substances in the gaseous mixture to be purified.
Consequently, the production of hydrogen by a PSA process (Pressure Swing Adsorption=Adsorption with Pressure Variation) has been widely studied.
However, most of the papers relating to this subject concern essentially the production cycles and the manner of conducting these cycles, so as to maximize the recovery and/or the purity of the hydrogen product, or else to reduce capital cost.
However, the choice of the adsorbent is delicate: it depends on the one hand on the nature of the mixture to be treated. As a general rule, the adsorbents are selected as a function of their ability to adsorb and desorb a particular compound.
Indeed, the PSA processes involve the use of pressure cycles.
In a first phase, the adsorbent bed ensures the separation of at least one constituent from the mixture by adsorption of this constituent on the bed of adsorbent.
In a second phase, the adsorbent is regenerated by lowering the pressure of the adsorbent beds operating in parallel.
U.S. Pat. No. 4,381,189 and French 2,330,433 illustrate particularly such an operation.
The elimination of the impurities contained in a flow of hydrogen takes place most of the time by means of at least two adsorbent beds disposed in series, namely conven- tionally an activated carbon bed and a zeolite bed.
In this connection can be cited WO-A-97/45363, which relates to a process for the purification of hydrogen base gaseous mixtures, polluted by various impurities, of which carbon monoxide and at least one other impurity selected from carbon dioxide and straight chain hydrocarbons, branched or cyclic, saturated or unsaturated, C1-8. The gas flow to be purified is placed in contact, in an adsorption zone, with a first adsorbent selective to carbon dioxide and C1-8 hydrocarbons, and a second adsorption which is a zeolite of the faujasite type exchanged at least 80% with lithium and whose Si/Al ratio is less than 1.5, to eliminate at least carbon dioxide (CO2). According to this document, the improvement afforded by the process is due to the use of a particularly effective zeolite, namely zeolite X exchanged with lithium.
Moreover, U.S. Pat. No. 3,150,942 teaches the use of a zeolite containing sodium or sodium and calcium cations, to purify a hydrogen flow.
Analogously, U.S. Pat. No. 4,477,267 discloses a process for the purification of hydrogen using a zeolite X exchange 70 to 90% with calcium cations and containing moreover an inert binder.
U.S. Pat. No. 4,957,514 discloses a process for the purification of hydrogen using zeolite X exchanged with 60 to 80% barium cations.
Moreover, U.S. Pat. No. 5,489,327 relates to the purification of gaseous hydrogen by contact with a zirconium hydride alloy.
Finally, JP-A-860146024 discloses a PSA process to purify impure gases using a mordenite type zeolite exchanged with lithium, on the production side and a zeolite one on the supply side.
Furthermore, U.S. Pat. Nos. 3,702,525, 3,986,849, 4,077,779, 4,153,428, 4,696,680, 4,813,980, 4,963,339, 3,430,418, 5,096,470, 5,133,785, 5,234,472, 5,354,346, 5,294,247 and 5,505,764 disclose PSA cycles of operation to produce hydrogen.
Conversely, certain documents emphasize that the adsorbent or the adsorbents used in a PSA process to purify hydrogen have little or no importance.
Thus, the paper Pressure Swing Adsorption, 1994, VCH publishers, D. M. Ruthvens, S. Farooq, K. S. Knaebel, page 238, teaches that xe2x80x9cas the selectivity for most of the impurities is high in comparison to that of hydrogen, any adsorbent whatever can be usedxe2x80x9d to purify hydrogen.
Analogously, according to U.S. Pat. No. 4,299,596, all the conventional adsorbents can be used to purify hydrogen, for example, activated carbon, silica gel, molecular sieves, such as zeolites, carbonated sieves.
Furthermore, U.S. Pat. No. 4,482,361 mentions the possibility of using no matter what suitable adsorbents, such as zeolitic molecular sieves, activated carbon, silica gels, activated alumina or the like.
Analogously, U.S. Pat. No. 4,834,780 teaches that adsorption can be carried out in all cases in which an adsorbent has been selected that is suitable for the separation process in question, for example, activated carbon, silica gel, alumina gel or molecular sieves.
It is thus apparent, in view of the prior art, that those skilled in the art conventionally consider that an improvement of the effectiveness of a PSA production for the production or purification of hydrogen can result only in an improvement of the production cycle or of the zeolitic material used, but that the influence of the adsorbent used is seldom of importance, which is to say that the adsorption has but little influence on the efficiency of the PSA process.
Starting with this, the problem which thus arises is to improve the PSA processes for the purification of hydrogen, which is to say to improve the efficiency of elimination of the impurities contained in a hydrogen flow to be purified.
The present invention seeks to solve this problem, which is to say that the process of the invention permits improving the conventional processes of the PSA type for separation of hydrogen or processes by adsorption with pressure variation.
The invention thus relates to a process for the purification of a gaseous hydrogen flow containing at least one gaseous impurity selected from the group consisting of carbon monoxide, water vapor, nitrogen, H2S, SO2, carbon dioxide, chlorine, lower C1-3 alcohols, ammonia and linear hydrocarbons, branched or cyclic, saturated or unsaturated, C1-8, in which the gaseous flow of hydrogen to be purified is contacted with at least one carbonated porous adsorbent characterized by a limit volume of adsorption (W0) and by an energy parameter (E0) according to the Dubinin theory, wherein:
0.18 ml.gxe2x88x921xe2x89xa6W0xe2x89xa61.50 ml.gxe2x88x921
and
15 kJ.molexe2x88x921xe2x89xa6E0xe2x89xa645 kJ.molexe2x88x921.
As the case may be, the process of the invention can comprise one or several of the following characteristics:
W0 is comprised between 0.20 ml.gxe2x88x921 and 1.20 ml.gxe2x88x921,
W0 is greater than or equal to 0.25 ml.gxe2x88x921,
W0 is greater than or equal to 0.30 ml.gxe2x88x921,
W0 is greater than or equal to 0.40 ml.gxe2x88x921, preferably W0 is greater than or equal to 0.50 ml.gxe2x88x921,
E0 is comprised between 20 kJ/molexe2x88x921 and 40 kJ.molexe2x88x921,
E0 is comprised is greater than or equal to 25 kJ.molexe2x88x921,
E0 and W0 are such that:
E0xe2x89xa647.5-12.5W0
and/or
E0xe2x89xa727.5-12.5.W0;
E0 and W0 are such that:
E0xe2x89xa642.5-12.5.W0
and/or
Exe2x89xa730-12.5.W0;
the porous carbonated adsorbent is selected from activated carbon, preferably activated carbon produced from coconut shell, pine bark, peat, lignite, anthracite, polymers, resins or other primarily organic materials.
the hydrogen flow is a synthesis gas or reforming gas, an electrolysis gas, a gas from ammonia cracking or alcohol cracking, such as methanol, preferably a gaseous mixture containing at least 70% hydrogen.
the porous carbonated adsorbent has pores having a size comprised between 0.4 nm and 4 nm, preferably between 0.5 nm and 2 nm.
it is of the PSA type, with one or several adsorbers, preferably several adsorbers. Conventionally, the cycles of a PSA process comprise successively, for each adsorber:
a substantially isobaric production phase at the high pressure of the adsorption cycle,
a regeneration phase of the adsorbent comprising at least one co-current decompression step with pressure balancing with another adsorber; a final step of countercurrent depressurization with evacuation of the residual gas; and generally, an elution step at the low pressure of the cycle, the elution gas coming from at least one second co-current decompression step of an adsorber; and
a repressurization phase comprising at least one step of pressure balancing with another adsorber and a step of final recompression by means of production gas.
Generally speaking, the cycles can comprise several balancing steps, total or partial, preferably with 1 or 4 balancing steps. The gaseous transfers can take place directly from adsorber to adsorber or by means of one or several gas storages. The recompression steps by balancing and by the production gas can be or not at least partially simultaneous, and can if desired comprise a partial repressurization by the gas feed. Supplemental steps of purging can be introduced, in particular if it is desired to recover and use another fraction of the gas to be treated, different from hydrogen. Moreover, the cycle can also comprise dwell times during which the adsorbers are isolated.
the adsorption pressure is comprised between 5 bars and 70 bars, preferably between 15 bars and 40 bars,
the desorption pressure is comprised between 0.1 bar and 10 bars, preferably between 1 and 5 bars,
the temperature of the flow of hydrogen to be purified is comprised between xe2x88x9225xc2x0 C. and +60xc2x0 C., preferably between +5xc2x0 C. and +35xc2x0 C.
the gaseous hydrogen flow is moreover placed in contact with a bed of zeolitic adsorbent, preferably a zeolite X, LSX or A.
the zeolite is preferably of the faujasite type exchanged with at least 70% lithium, a zeolite of the faujasite type whose Si/Al ratio is comprised between 1 and 1.2 and is preferably equal to 1.
the volume ratio of the porous carbonated adsorbent to the zeolite adsorbent is comprised between 10/90 and 90/10, preferably between 50/50 and 80/20.
The invention moreover concerns a porous carbonated adsorbent, characterized by a limit adsorption volume W0 and by an energy parameter E0, according to the Dubinin theory, wherein:
0.18 nml.gxe2x88x921xe2x89xa6W0xe2x89xa61.50 ml.gxe2x88x921
and
15 kJ.molexe2x88x921xe2x89xa6E0xe2x89xa645 kJ.molexe2x88x921,
preferably
E0xe2x89xa647.5-12.5.W0
and/or
E0xe2x89xa727.5-12.5.W0.
Thus, the inventors of the present invention have shown that, in a surprising manner, a substantial improvement of the efficiency of a PSA process for the purification of a flow of hydrogen as to the impurities it contains, can be achieved thanks to a judicious choice of the carbonated adsorbent, which is to say of the activated carbon, used in the PSA process, in particular when said activated carbon is used in association with a bed of another adsorbent, preferably a bed of zeolite.
Thus, although it is usual to use particles of activated carbon to eliminate certain of the impurities contained in hydrogen flows, up until now it was not known that the microporous structure of the microporous activated carbon plays an important role in the performance of an adsorption process of the PSA type to purify or separate a flow of gaseous hydrogen.
However, there is a microporous structure which ensures optimum performance in terms of yield of hydrogen recovery.
The structure of the micropores defines the intensity of the adsorptive forces as well as the volume used.
Thus, an activated carbon is a non-crystallized compound conventionally obtained by careful heating and oxidation of a carbonated precursor which can be of vegetable origin (coconut, pine bark), mineral (peat, lignite, anthracite) or else synthetic (polymer).
It is characterized by at least two parameters which are the total porous volume and the intensity of adsorption which depends among other things on the size of the pores.